1. Field of the Invention
The present invention relates, generally, to a printed circuit board (PCB) having embedded capacitors using a hybrid material and a method of manufacturing the same. More particularly, the present invention relates to a PCB having embedded capacitors, in which a hybrid material, comprising liquid crystal polymer and high-frequency ceramic powder dispersed therein, is formed in a sheet shape for use in a capacitor layer, which is then embedded in the PCB, thereby decreasing the variation of capacitance depending on temperature and reducing signal transmission loss by virtue of a low dissipation factor, compared to conventional PCBs having embedded capacitors, and to a method of manufacturing such a PCB.
2. Description of the Related Art
In general, capacitors store energy in the form of an electric field. When a DC voltage source is applied to a capacitor, the capacitor is charged but the current flow stops. On the other hand, if an AC voltage source is connected to a capacitor, the current flows through the capacitor depending on the frequency of the applied AC signal and the value of the capacitor while the capacitor is charged and discharged.
Thus, the capacitor, having the above properties, acts as a passive component essential for use in a variety of purposes, for example, coupling and decoupling, filters, impedance matching, signal matching, charge pumps and demodulation in electric and electronic circuits, such as digital circuits, analog circuits, and high frequency circuits. Further, the capacitors, which are manufactured in various forms, such as chips or disks, have been used in the state of being mounted on PCBs.
The capacitor in the electronic circuit is largely classified into two kinds, depending on the capacitance and temperature stability thereof, that is, a capacitor such as B (A) and F type MLCC (Multi-Layer Ceramic Capacitor), having low temperature stability and high capacitance, and a capacitor such as C type MLCC, having low capacitance and high temperature stability. The former is mainly used for decoupling and bypassing, whereas the latter is used for signal matching and impedance matching.
Although passive components such as capacitors have been manufactured in various forms such as chips or disks to be mounted on PCBs to date, electronic devices have recently been required to be miniaturized and complicated, and thus, the areas for mounting the passive components on the PCB are decreasing. Further, while frequencies increase in accordance with high speed electronic devices, parasitic impedance is generated by the conductors, solders, etc., between the passive components and the IC, thus causing several problems. In order to solve the problems, various attempts have been made to embed the capacitor in the PCB, mainly led by the manufacturers of PCBs and electric and electronic components.
However, since most materials for PCBs having embedded capacitors developed by the manufacturers to date have unstable capacitance depending on the temperature and humidity, they have been limitedly studied only for use in decoupling and bypassing.
In this regard, FIGS. 1A to 1E sequentially illustrate a conventional process of manufacturing a PCB having embedded polymer thick film capacitors. According to the conventional process, a polymer capacitor paste is applied on a substrate and then dried using heat (or cured), to realize the PCB including embedded polymer thick film capacitors, which is specifically described with reference to the drawings.
In a first step, a copper foil of an inner layer for a PCB 42 including FR-4 is coated with a dry film, exposed, developed, and then etched, to form anodic copper foils 44a, 44b, cathodic copper foils 43a, 43b, and clearances therebetween (FIG. 1A).
In a second step, capacitor pastes 45a, 45b, composed of a polymer that contains ceramic powder having a high dielectric constant, are applied on the cathodic copper foils 43a, 43b using a screen printing process, and are then dried or cured (FIG. 1B). Herein, the screen printing is performed by passing a medium such as ink through a stencil screen using a squeeze, thereby transferring a pattern to the surface of a substrate.
At this step, the clearances between the anodic copper foils 44a, 44b and the cathodic copper foils 43a, 43b are covered with the capacitor pastes 45a, 45b. 
In a third step, a conductive paste including silver or copper is formed into anodes 46a, 46b using a screen printing process, and is then dried or cured (FIG. 1C).
In a fourth step, the capacitor layer, subjected to first to third steps of the inner layer of the PCB 42, is inserted between insulating layers 47a, 47b, followed by being laminated (FIG. 1D).
In a fifth step, through holes and laser blind via holes 49a, 49b are formed through the laminate, whereby the capacitor present in the inner layer of the PCB is connected to positive terminals 51a, 51b and negative terminals 50a, 50b of IC chips 52a, 52b mounted outside the PCB, thus acting as an embedded capacitor (FIG. 1E).
Likewise, there are disclosed methods of manufacturing an embedded discrete type capacitor by coating a PCB with a ceramic-filled photo-dielectric resin, which have been patented by Motorola Co. Ltd., USA. The above method comprises applying a photo-dielectric resin containing ceramic powder on a substrate, laminating copper foils on the resin layer to form upper electrodes and lower electrodes, forming a circuit pattern, and then etching the photo-dielectric resin, to realize the discrete capacitor.
Further, there are proposed methods of fabricating an embedded capacitor by separately including a dielectric layer having capacitance properties in the inner layer of a PCB, so as to be used instead of a decoupling capacitor mounted on the PCB, which have been patented by Sanmina Co. Ltd., USA. In this method, the dielectric layer, including power electrodes and ground electrodes, is incorporated in the inner layer of the PCB, to obtain a power-distributed decoupling capacitor.
Various processes are under study to achieve the above techniques, methods of embodying each process being varied.
In this regard, U.S. Pat. No. 5,079,069, granted to Howard et al., discloses a capacitor laminate for use in capacitive PCBs and methods of manufacture, in which the concept of ‘borrowed capacitor’ is used, to manufacture the PCB including a structurally rigid capacitor thin film layer composed of two conductive layers and a dielectric layer sandwiched between the two layers, which is in operative connection with a large number of devices.
Also, U.S. Pat. No. 5,010,641, granted to Sisler et al., discloses a method of making a multilayer PCB to eliminate the need for the by-pass capacitor by providing one or more fully cured power-ground plane sandwich components which are laminated together with other partially cured component layers of the PCB and circuit pattern-formed components.
According to the conventional techniques, the thick film is formed mainly of epoxy resin and ceramic powder. In the case of the thin film, ceramic, such as barium titanate, is formed into a thin film through a thin film preparation process, which has been invented for embedding a capacitor for use in decoupling and bypassing. Although such a thin film has relatively high capacitance, it entails large variation of capacitance depending on the temperature, and also a high dissipation factor, and is therefore unsuitable for use in embedding the capacitor which has been presently mounted on the PCB for signal matching and impedance matching in high frequency circuits.